Colloidal silicas, also known as silica sols or silica hydrosols, are aqueous systems with very small silica particles which find use in a variety of fields, among other things dependent on the particle size. In the production of paper, silica based sols with very small, colloidal, anionic silica particles have found an increasing utilization during the last two decades. Colloidal silicas are hereby used as additives to the stock in combination with cationic or amphoteric polymers, mainly for improving retention and dewatering in the production of paper. The preparation and use of colloidal silicas are disclosed, for example, in European Patent 0041056, PCT Publication Nos. WO 86/00100 and WO 86/05826, U.S. Pat. Nos. 5,603,805, 6,372,089, and 6,486,216, the contents of all of which are incorporated herein by reference for all purposes.
The colloidal silicas useful in paper production are of the type which have discrete colloidal particles with a particle size usually of from about 4 to about 7 nm, a specific surface area of from about 300 to about 1000 m2/g, and an S-value of from about 20 to about 40%. Colloidal silica products are commercially available from a variety of suppliers, for example, Nalco Chemical Company and EKA Nobel, AB.
For example, U.S. Pat. No. 5,603,805 discloses silica sols having an S-value within the range of from 15 to 40 percent comprising anionic silica particles, said silica particles being non-aluminum modified and having a specific surface area within the range of from 300 to 700 m2/g. The silica sols disclosed in U.S. Pat. No. 5,603,805 have a SiO2 content of from about 5 to 7.5 percent by weight.
At the present, methods for producing silica sols mainly utilize continuous producing methods. For example, U.S. Pat. No. 5,503,820 discloses a method for producing low concentration polysilicates microgels and the corresponding apparatus. In the apparatus of the said patent, a solution of water soluble silicate and a strong acid are supplied to a mixing device simultaneously. After the silicate is completely reacted with the strong acid in the mixing device, the resulting mixture is transported into a receiving tank. Meanwhile, water as a diluent is also supplied to the receiving tank, so that to dilute the resulting mixture to obtain a final diluted colloidal silica product. U.S. Pat. No. 5,853,616 also discloses a similar apparatus for producing colloidal silica.
Although above apparati may mix raw materials relatively uniform and can continuously produce colloidal silica products, but the quality of the products suffer instability. These products are unacceptable in terms of colloidal silica supplied for faster and greater paper making machine. In addition, in these production apparati, caustic sodium hydroxide is used to clean the reaction vessel each time after the reactions, which can cause corrosion of the reaction vessel, and can consequently increase the operation cost and bring environmental concerns.
There is still a need for a colloidal silica having a high solids content and a low S-value. In addition, while there are many multi-component programs used to improve retention and drainage performance in papermaking, there is still a need for new programs used to improve retention and drainage performance in especially the faster and bigger paper machines that are being built. Moreover, there is also a need for a new production apparatus which can stably produce activated colloidal silica having high solid content and low S-value, as well as having improved retention and drainage property.